I am concerned about the letter to the editor written in PenBayPilot.com dated Dec. 27, 2021 by Bob Kohl, entitled Optimizing our Immune Systems to prevent severe COVID-19.    As a licensed naturopathic doctor for more than 20-years, my colleagues and I often encourage adequate vitamin D supplementation when blood testing reveals a deficiency.  I encourage everyone to make healthy choices when it comes to support of our immune system, such as eating a diet rich in fruits, vegetables, healthy protein sources and drinking plenty of water, getting seven-plus hours sleep per night, limiting sugar and sweets, alcohol and stress and getting plenty of sunlight and fresh air and exercise.    While I do appreciate Mr. Kohl putting the spotlight on the importance of vitamin D3 and immune regulation, however, the title implies that by simply taking vitamin D people can avoid severe COVID-19.    I would retitle the article to read: Interesting information on the role of Vitamin D and COVID-19 immune response.    If someone is concerned that they have inadequate levels a simple blood test can be done by your ND or PCP.  In my patient base, the goal of the blood test would be between 50-65 ng/mL.    Keep in mind that there are some medical conditions that people may have making taking too much D3 can be detrimental.    Thank you,   Dr. Barb MacDonald, ND, LAc (Rockport).